1. Field of the Invention
The present invention relates to an impact energy absorbing structure which is used to absorb impact energy on the heads of passengers during collisions in vehicles, for example.
2. Related Background Art
In order to ensure higher levels of passenger protection in the interior of vehicles such as automobiles, the recent trend has been toward the establishment of increasingly stringent standards regarding passenger safety measures and the like. In the United States, in particular, the Federal Motor Vehicle Safety Standards have been strengthened (FMVSS 201U), resulting in more stringent regulations regarding measures for protecting passenger heads. In FMVSS 201U, dummy heads (mass 4.54 kg) referred to as free motion headforms (FMH) are used as the colliding object, which is caused to collide on an interior part (impact energy absorbing structure) at 24 km/h, and the total acceleration a (t) at the FMH center of gravity is measured.
FIG. 1 illustrates an image from such an impact test, showing a schematic example of the shape of such an impact energy absorbing structure. FIG. 2 illustrates an image of the deformation characteristics of the structure during the impact test, where the headform acceleration is shown relative to the deformation of the structure. In the figure, the dashed line shows the results obtained with a conventional structure, where the acceleration is greater in the second half than in the first half of deformation. Because the product of the headform acceleration and the structure deformation expresses the collision energy (per kilogram) absorbed by the structure, the distribution of absorbed energy is greater in the second half than in the first half.
In this test, the total acceleration a(t), measured from a sensor is treated with Equations (5) and (6), giving HIC(d). HIC(d) is stipulated at no more than 1000 in FMVSS 201U.
                    HIC        =                  MAX          ⁢                      {                                                            [                                                            1                                              (                                                                              t                            2                                                    -                                                      t                            1                                                                          )                                                              ⁢                                                                  ∫                                                  t                          1                                                                          t                          2                                                                    ⁢                                              a                        ⁢                                                  ⅆ                          t                                                                                                      ]                                25                            ⁢                              (                                                      t                    2                                    -                                      t                    1                                                  )                                      }                    ⁢                      (                                                            t                  2                                -                                  t                  1                                            ≤                              36                ⁢                                                                  ⁢                msec                                      )                                              (        5        )                                          HIC          ⁡                      (            d            )                          =                                            0.75446              ×              HIC                        +            166.4                    <          1000                                    (        6        )            The terms in brackets in Equation (5) express the mean, between t1 and t2, of the total acceleration measured by a triaxial accelerometer sensor, and HIC is the standard for assessing passenger head protection performance.
To study what type of structure will permit the HIC(d) to be minimized, most studies have thus far been based on the time-acceleration behavior of the colliding object. As illustrated by the dash-dot line in FIG. 2, cases in which the time-acceleration behavior is in the form of a rectangular wave, that is, cases in which the impact absorption energy is uniformly distributed over time (and thus uniformly distributed over deformation), are reportedly the most effective. As a result, a major issue in the design of actual products has been how to make the deformation behavior (time-acceleration behavior of colliding object) more closely resemble a rectangular wave.